import pygame
from pygame.sprite import Sprite

class Alien(Sprite):
	"""表示单个外星人的类"""
	def __init__(self, ai_game):
		"""初始化外星人并设置其初始位置"""
		super().__init__()
		self.screen = ai_game.screen
		self.settings = ai_game.settings

		#加载外星人图像并设置其rect属性
		self.image = pygame.image.load('images/alien.bmp')
		self.rect = self.image.get_rect()

		# 每个外星人最初都在左上角附近
		self.rect.x = self.rect.width
		self.rect.y = self.rect.height

		#储存外星人的精准水平位置
		self.x = float(self.rect.x)

	def check_edges(self):
		"""如果外星人位于屏幕边缘，就返回True"""
		screen_rect = self.screen.get_rect()
		if self.rect.right >= screen_rect.right or self.rect.left < 0:
			return True

	def update(self):
		"""向右或向左移动外星人"""
		self.x += (self.settings.alien_speed * self.settings.fleet_direction)
		self.rect.x = self.x

